When a metallic material is left in the air, in general, water in the atmosphere adsorbs to its surface, and a corrosion factor such as SO2, NOx, and Cl, also, flows in the air and adsorbs to its surface. The metallic material under such an environment will corrode by the coexistence of water and the corrosion factor. In order to reduce such corrosion of metallic materials by water and corrosion factor, various surface treatment techniques have been developed.
The corrosion prevention method for metal can be in general roughly divided into two types: one is a method by coating, and the other is a method used without coating such as chromate treatment on a galvanized material. In the two types of corrosion prevention methods, surface treatment is absolutely performed.
When a coating is applied on a steel material, for example, a method in which cleaning treatment is performed on the steel material, and zinc phosphate treatment is then performed thereon and a method in which in the above method chromate treatment or phosphoric-acid chromate treatment is further performed after the zinc phosphate treatment are known.
Steel materials having various kinds of galvanization may be used without a coating. In such a case, a method is known in which cleaning treatment is performed on a steel material, and then without performing zinc phosphate treatment, chromate treatment or phosphoric-acid chromate treatment is performed.
In such a case aiming at using a metallic material in the bare condition without a coating, general surface treatment processes to be performed on a metallic material include degreasing treatment, galvanization, acid cleaning, chemical conversion treatment, and drying in this order. The above chromate treatment or the like is performed as the “chemical conversion treatment” in the above processes.
Agents for use in such chromate treatment and phosphoric-acid chromate treatment are low in price and provide a resultant coating with good corrosion resistance. However, hexavalent chromium contained in a treatment liquid is harmful and considering recent environmental restrictions, chromate treatment and phosphoric-acid chromate treatment tend to be refrained from being used.
In recent trends, trivalent chromium treatment using a harmless trivalent chromium compound has been proposed as a surface treatment method in which, for example, a zinc-based metallic material such as an electrogalvanized steel sheet is provided with corrosion resistance without using harmful hexavalent chromium.
The trivalent chromium treatment includes mainly electrolytic trivalent chromate treatment, dry-in-place type trivalent chromate treatment, and reaction type trivalent chromate treatment (See Patent Documents 1 to 3, for example). Some of these pieces of trivalent chromate treatment show performance exceeding the hexavalent chromate treatment and are regarded as a very likely technique alternative to the harmful hexavalent chromate treatment.
However, although trivalent chromium itself for use in the trivalent chromate treatment is harmless, it is still a chromium compound, and may be changed into hexavalent chromium through an oxidation reaction. With recent growing consciousness about environmental problems worldwide, there is a strong movement under way to avoid use of substances which may have a detrimental effect on the environment even in a small extent, there is a possibility that even use of trivalent chromium will be banned in the future.
A chemical conversion technique without using chromium (non-chromium technique) on nonferrous materials other than the zinc-based metallic material such as an aluminum-containing material has been studied for relatively a long time from the standpoint of anti-corrosiveness. For example, an aluminum-containing material for use in food and drink has been studied from around 1990, and has been in practical use.
A representative chemical conversion treatment liquid containing no chromium on the aluminum-containing material is disclosed in Patent Document 4. The treatment liquid contains zirconium or titanium or their mixture, phosphate, and fluoride, and is an acidic water-based surface treatment liquid with a pH of about 1.0 to 4.0. Performing treatment using this treatment liquid forms a precipitated coating having a phosphorous compound of zirconium or titanium as a main component on the surface of an aluminum-containing metallic material.
However, it is known that the precipitated coating having a phosphorous compound of zirconium or titanium as a main component is very thin, and if it is thickened, cracks occur on its surface. When it is actually used for printing on beverage cans, therefore, the coating is controlled to have low coating weight.
Furthermore, surface treatment methods which use no chromium and provide the surface of metallic materials with good corrosion resistance are disclosed in Patent Documents 5 to 9.    Patent Document 1: Japanese Patent Laid-open No. 5-009746    Patent Document 2: Japanese Patent Laid-open No. 2000-282255    Patent Document 3: Japanese Patent Laid-open No. 2003-166074    Patent Document 4: Japanese Patent Laid-open No. 56-136978    Patent Document 5: Japanese Patent Laid-open No. 2004-190121    Patent Document 6 Japanese Patent Laid-open No. 2004-218072    Patent Document 7: Japanese Patent Laid-open No. 2004-218074    Patent Document 8: Japanese Patent Laid-open No. 2004-218075    Patent Document 9: Japanese Patent Laid-open No. 2005-264230